Lower-limb influence on sitting balance while reaching forward.

To test the hypothesis that the lower limbs contribute to sitting balance when a person reaches forward, we measured the limits of forward reach in 20 healthy volunteers. While sitting on the buttocks alone (ischial support) or with the thighs also supported and with both feet on, one foot off, or both feet off the floor (in a balanced order), each subject reached as far forward as possible at table height at 0, 15, 30, and 45 degrees to the sagittal plane. With full thigh support the reaches at all angles were greatest with both feet on the floor, and decreased progressively with one foot off (when reaching toward the foot off the floor) and both feet off. Thigh support permitted significantly greater reach than ischial support at all angles with both feet off the ground, but not in other settings. These results have implications for seat design and when considering prostheses for patients unlikely to walk.